The retort process sterilizes packaged food in its container to create a shelf-stable product. Polymeric films used in retort packaging must withstand high heat (typically 121° C. and higher) and high humidity. The current film structure for retortable food packaging contains at least three functional layers: sealant layer, barrier layer and tie layer in between. The sealant layer is generally polyolefin, preferably polypropylene for heat resistance. The barrier layer is polyamide (PA) or ethylene vinyl alcohol (EVOH) or polyvinylidene chloride (PVDC), preferably PA which maintains mechanical strength and oxygen barrier properties during the retort process. The tie layer creates interlayer adhesion between the two incompatible layers. Typically, it is maleic anhydride (MAH) functionalized polyolefins, specifically MAH-g-PP or MAH-g-PE. Since the sealant layer is preferably polypropylene (PP), the choice of the functionalized polyolefin is generally MAH-g-PP for tie layer. However, the MAH-g-PP process degrades PP and causes significant melt flow rate increase and difficulty in viscosity matching for film co-extrusion. In addition, washing is needed to remove PP oligomers and therefore cost to produce is high.
It would be desirable to provide an alternative tie layer that maintains the required seal strength and does not have haze or shrinkage issues and allows for coextrusion.